Modified sol–gel synthesis greatly improves textural properties and photocatalytic performance of Cu-doped phase-pure ZnAl2O4

Abstract

This work addresses the dual challenges of sustainable H₂ production and wastewater treatment. We developed high-performance Cu-doped ZnAl₂O₄ (ZAO) spinels through a modified sol–gel citrate route. Calcination under N₂ with a limited O₂ supply harnesses residual citrate-derived carbon to confine crystal growth, yielding phase-pure nanocrystalline Zn_1−xCu_xAl₂O₄ materials (x = 0.01–0.2) with uniform mesopores and a fourfold increase in surface area versus conventional sol–gel synthesis. Elemental mapping confirmed uniform Cu incorporation, while ICP-MS confirmed the target stoichiometry. XPS confirmed an increase in oxygen vacancy concentrations with increasing Cu doping, and bandgap engineering (3.95 to 2.33 eV) enabled broad visible-light absorption at the higher Cu contents. Charge carrier dynamics were studied using time-resolved photoluminescence spectroscopy and revealed drastically suppressed radiative recombination and extended exciton lifetime from ~3 ns in undoped ZAO (or conventional ZAO Air) to >19 ns for Cu-doped variants. The evolution of these synergistic properties with Cu-loading improved the photocatalytic functionality of these materials: Cu-doped ZAO exhibited dramatically enhanced Congo Red photodegradation, while 5–20% Cu-loaded compositions achieved record hydrogen evolution rates among previously reported phase-pure aluminate spinels. The 20% Cu-ZAO photocatalyst showed the best performance under pure visible light (>400 nm) both with and without sacrificial agents, demonstrating excellent applicability for solar-driven water splitting. This work establishes a tunable platform for developing dual-function mesoporous photocatalysts with high surface area for advancing technology in sustainable energy and environmental remediation.